Various types of circuits have been employed to determine the status or condition of the switch, and provide an indication of the switch condition. By way of example, U. S. Pat. No. 4,658,249, entitled "Data Communication System With Key Data Bit Denoting Significance of Other Data Bits", which issued Apr. 14, 1987 to William R. Vogt, and is assigned to the assignee of this application, includes a generalized showing of a switch state determination circuit. An improvement to that determination circuit of the '249 patent was subsequently described and claimed in an application entitled "Switch Monitoring Arrangement With Remote Adjustment Capability Having Debounce Circuitry For Accurate State Determination", filed Apr. 29, 1988, Ser. No. 188,323, which issued Aug. 1, 1989 as U.S. Pat. No. 4,853,685 in the name of William R. Vogt, and is assigned to the assignee of the present application. In addition to the remote adjustment feature described in the '685 patent, a two-step arrangement is provided in which each time the switch changes state, a status signal connoting the actual switch state is produced. In addition the '685 patent teaches the production of a confirmation signal upon verifying the presence of the status signal for some preset time. This operation, coupled with the remote adjustment feature, produced a significant step forward in this art.
It has since been observed that the switch may oscillate rapidly, producing changing status signals, without ever remaining in the given state for the preset time period so that a confirmation signal can be provided, to verify that the switch is indeed in an appropriate state and has been there for the preset time. Such operation would appear abnormal to the usual system, because neither a trouble nor an alarm, nor any other unusual signal, is generated by the rapid variation of the switch and the status signals. Nevertheless such operation is undesirable, as an intermittent connection or some other aberration could provide this rapid oscillation between states without being detected by alarm systems presently in use.
It is therefore a principal consideration of the present invention to provide a system for detecting irregular operation of a switch state monitoring system, particularly monitoring arrangements in which switch status is first indicated and subsequently confirmed by another circuit.